At the present time, steam ovens are used to sterilize seed beds such as those used for example, in growing mushrooms. Such steam ovens have a single-walled structure which causes cooling in the chamber. Thus, external air adversely affects the temperature of the steam, with the result that the articles contained within the oven are sterilized unevenly. Furthermore, since the steam is used only once in such ovens, and the condensate is not recycled for reuse, a high-pressure bactericidal oven which does not utilize an exterior water supply cannot be used for long periods of treatment. On the other hand, in the case of a bactericidal oven used at atmospheric pressure where water can be supplied from an exterior source, long periods of treatment are possible but the cost of heating water is high.